"It is important to save the country from the grave economic crisis," it said in a brief statement.
And the second-largest opposition party, the Sri Lanka Freedom Party (SLFP), said it would join the cabinet.
Even so, thousands of protestors remained camped outside the sea-front office of 73-year-old president Gotabaya Rajapaksa, whose brother Mahinda quit as premier last week after political violence killed at least nine people.
Shortages of food, fuel and medicines, along with record inflation and lengthy blackouts, have brought severe hardships to Sri Lankans, in the worst financial crisis since independence from Britain in 1948.
Wickremesinghe's appointment last week -- his sixth turn as prime minister -- has so far failed to quell public anger at the government for bringing Sri Lanka to the brink of economic collapse.
Troops patrolled the streets as consumers queued up for scarce supplies and the government announced that a six-hour night curfew will be re-imposed from Monday after a 24-hour break for a religious holiday.
Wickremesinghe said on Monday that Sri Lanka had no dollars to finance essential imports with three oil tankers were waiting off Colombo to be paid before they would unload.
He added that the country has run out of 14 essential drugs, including anti-rabies vaccines. The state's health ministry has not paid its suppliers of medicines for four months and has now been blacklisted, he added.
He also warned that fuel and electricity tariffs will be raised substantially and his government will also sell off its loss-making national airline.
However, he urged people to "patiently bear the next couple of months" and vowed he could overcome the crisis.
Wickremesinghe has struggled to form a "unity government" and a cabinet swearing-in scheduled for Monday afternoon was pushed back as talks continued on sharing portfolios.
Four ministers were sworn in on Saturday, all from Rajapaksa's Sri Lanka Podu Jana Peramuna (SLPP) party.
But there is no finance minister yet, and it is widely expected that Wickremesinghe will retain the crucial portfolio to lead ongoing negotiations with the IMF for an urgent bailout.
The new prime minister held talks Sunday with World Bank and Asian Development Bank representatives in Colombo on medicine, food, fuel and fertiliser supplies, his office said in a statement.
Long queues stretched outside the few fuel stations that were still open on Monday as motorists waited for rationed petrol.
Heavily armed troops were patrolling the streets with a state of emergency still in effect after at least nine people were killed in violence last week.
Police said over 350 people have been arrested in connection with last week's mob violence.